1. Field of the Invention
The present invention relates to a hydraulic control valve system suitable for a system for driving a plurality of hydraulic actuators by one pump.
2. Description of the Prior Art
A construction machine involves the use of a large capacity hydraulic pump. A plurality of actuators are driven with a discharge oil from this hydraulic pump. For instance, in a power shovel, the single large capacity hydraulic pump drives a turning hydraulic motor, a left traveling motor, a right traveling motor, a boom cylinder, an arm cylinder and a bucket cylinder.
In this hydraulic system, direction switching valves are connected between the signal hydraulic pump and the respective actuators. It is a common practice that a quantity of oil sent to the actuator is compensated by a pressure compensation valve to restrain variations in operating velocity of the actuator due to fluctuations in load. In the prior art, however, a control pressure of the pressure compensation valve is set depending on a spring property of a spring. For this reason, there arise such problems that a control pressure difference is sufficiently secured with difficulty due to a lack of pump discharge quantity or a difference in load pressure; and the operating velocities of the plurality of actuators are easily brought into an ill-balanced state.
Proposed as a counter measure for the problems in Japanese Patent Application Laid-Open Publication No. 11706/1985 was such an arrangement that the throttle opening of the pressure compensation valve is controlled not by a spring force but by a pressure difference between a pump discharge pressure and a signal pressure from a shuttle valve. More specifically, according to this prior art, the pressure compensation valve is provided on the upstream side of the direction switching valve. A load of a pressure (bridge pressure) reaching the direction switching valve is applied on the closing side of pressure compensation valve. A load pressure of the actuator is applied on the opening side thereof. Separately from this set of opening/closing pressures, the maximum load pressure (selected by the shuttle valve) of the actuator which is on the operation is applied on the closing side of the pressure compensation valve. A load of a pump discharge pressure is put on the opening side thereof.
This prior art (hereinafter referred to as the prior art 1), however, presents only hydraulic circuitry. As a concrete system, there was expected to the utmost such a mode that the pressure compensation valves each having an independent structure, the shuttle valve and the direction switching valve are connected to each other through external pipes. For this reason, the system becomes complicated and increases in size.
Thereafter, Japanese Utility Model Laid-Open Publication 150201/1989 (hereinafter referred to as the prior art 2) was proposed. This prior art 2 is an embodied version of the prior art 1. The prior art 2 is superior in terms of the arrangement that one valve body skillfully incorporates the direction switching valve, the pressure compensation valves and the shuttle valve. The prior art 2 is still, however, accompanied with a problem in which load pressure introducing passages to the pressure compensation valves and the shuttle valve are intricate, and manufacturing/assembling operations are therefore troublesome. Whether in the prior art 1 or in the prior art 2, the actuator load pressure (opening-side pressure) confronts the pressure (closing-side pressure) on the upstream side of the notch of the direction switching valve. On the other hand, the pump discharge pressure (opening-side pressure) confronts directly the maximum load pressure (closing-side pressure) selected by the shuttle valve. The throttle opening is controlled based on a pressure difference therebetween. As a result, a degree of freedom to control the throttle opening of the pressure compensation valve is poor. It is difficult to individually match with requirements of various operating conditions for every actuator.
The following was proposed as a countermeasure for this in Japanese Patent Application Laid-Open Publication No. 134402/1990. Based on this prior art (hereinafter referred to as the prior art 3), the passages for connecting the above-mentioned three types of valves to each other are composed of single internal passages. The advantages thereof are such that the whole valve unit can be made compact, and the multi-valve unit is easily attainable. As a closing-side pressure element of the pressure compensation valve, the maximum load pressure selected by the shuttle valve is not directly employed. Alternatively used is the external control pressure set corresponding to a differential pressure between the maximum load pressure selected by the shuttle valve and the pump discharge pressure. Hence, there are obtained merits in which the degree of freedom for pressure compensation control is high, and a good controllability of the pressure compensation is obtained even when the maximum load pressure fluctuates.
In the prior art 3, however, if the external control pressure which controls the maximum load pressure of the actuator biases a balance piston of the pressure compensation valve to the closing side, the balance piston abruptly drops down. In contrast with this, an opening-side chamber of the balance piston undergoes directly the fluctuations in load pressure of the actuator. This causes such problems that when a minute flow rate is controlled by the pressure compensation valve--i.e., when the throttle opening of the pressure compensation valve is minute, a hunting phenomenon takes place; and the flow rate can not be controlled well.